1. Field of the Invention
This invention relates to a sub connectable in a drill string for facilitating entry and exit of a flexible line between the exterior and interior of the drill string. The flexible line may be, for example, a logging cable or a flexible tube.
2. Description of the Prior Art
Side-entry subs have at least two applications in the drilling art. One such application is for a steering system used with a mud motor. In such an application a wireline is suspended along the outside of the drill string, enters the string through the body, and is secured therein through a side-entry packoff often disposed near the drilling head. For example, U.S. Pat. No. 4,388,969 to Marshall et al., discloses a side-entry sub which connects a steering tool with surface monitoring equipment by means of a wireline thereby permitting continuous steered drilling. The apparatus of the Marshall patent may also allow entry of the wireline from outside the drill string to its interior while logging a highly deviated hole. The Marshall patent includes a passage through the wall of the body of the sub and provides a means for sealing a cable running through the passage with respect to the pressurized drilling fluid flowing through a longitudinal bore of the sub.
A disadvantage of the apparatus of the Marshall patent is that a wireline cable, entering the pipe string via the sub or exiting the pipe string during logging of the well, faces extremely high frictional forces due to the frictional contact of the cable on angular bends and turns in the passage. The force required to overcome such frictional drag on the logging cable may be so great as to prevent the effective logging of the well due to either exceeding the tension capability of the cable or the pulling capability of a logging winch. The armour of the logging cable may experience rapid wear and failure due to the constant frictional wear.
Another example of a side-entry sub is that disclosed in French patent publication No. 2,502,236 of Sept. 24, 1982, and assigned to Institut Francais du Petrole (I.F.P.). The side-entry sub disclosed in the I.F.P. patent publication includes a detachable sheath which is inserted in an inclined recess formed in the wall of the tubular body of the sub. The sheath has a drill hole provided through it. A portion of the drill hole has a diameter substantially equal to that of the logging cable and another portion of it accommodates a sealing apparatus. The sheath extends on the inner side of the sub into a member for guiding the cable at the outlet of the sheath so as to limit the curvature of the cable.
The I.F.P. apparatus suffers from the same sort of disadvantage as does the Marshall et al. patent in that frictional forces of a non-movable guide may also produce unacceptably large drag forces during the logging of the well as the logging cable is spooled out of the pipe string and through the sub.